In case you create wrong version or just don't need it anymore in your banner set, then you can of course delete it.
1. In the banner set where the version you wish to delete is, select that specific version and then click on Edit.
2. Once the popover has appeared when "Edit" has been clicked, simply click on Delete to delete the version. It's as simple as that. Please note that if the banner is used live on a website, then this banner will be rendered as blank instead (not displayed).